[Sync] Add history delete directive type

sync/protocol/nigori_specifics.proto

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Sync protocol datatype extension for nigori keys.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
@@ skipping 33 matching lines @@
   // optional bool deprecated_encrypt_autofill = 6;
   // optional bool deprecated_encrypt_themes = 7;
   // optional bool deprecated_encrypt_typed_urls = 8;
   // optional bool deprecated_encrypt_extensions = 9;
   // optional bool deprecated_encrypt_sessions = 10;
   // optional bool deprecated_encrypt_apps = 11;
   // optional bool deprecated_encrypt_search_engines = 12;
 
   // Booleans corresponding to whether a datatype should be encrypted.
   // Passwords are always encrypted, so we don't need a field here.
+  // History delete directives need to be consumable by the server, and
+  // thus can't be encrypted.
   optional bool encrypt_bookmarks = 13;
   optional bool encrypt_preferences = 14;
   optional bool encrypt_autofill_profile = 15;
   optional bool encrypt_autofill = 16;
   optional bool encrypt_themes = 17;
   optional bool encrypt_typed_urls = 18;
   optional bool encrypt_extensions = 19;
   optional bool encrypt_sessions = 20;
   optional bool encrypt_apps = 21;
   optional bool encrypt_search_engines = 22;
@@ skipping 39 matching lines @@
   // The time (in epoch milliseconds) at which the keystore migration was
   // performed.
   optional int64 keystore_migration_time = 32;
 
   // The time (in epoch milliseconds) at which a custom passphrase was set.
   // Note: this field may not be set if the custom passphrase was applied before
   // this field was introduced.
   optional int64 custom_passphrase_time = 33;
 }